Automatically Resize Blogger Images

Automatically Resize New post Only:
Open up the Blogger dashboard and go to 'Template' > 'Customize'. You'll see the option for 'Advanced', click on it and find your way to the 'Add CSS' tab... paste the following CSS code inside the empty box:

.post-body img {
max-width: 100%;
max-height: auto;
display: block;
margin: auto; }

Automatically Resize New post & old also :
Remember that when you make changes to the CSS of the blog's Template, these changes will affect every post previously added, using the "Original size" dimension option.

.post-body img {
max-width: 100%;
max-height: auto;
display: block;
margin: auto; }
box-shadow: none;
border: none;
padding: 0px; 
}